1. Field of the Invention
The subject invention generally pertains to mobile service rigs for wells and more specifically to a mobile service rig that includes an engine powering a hoist.
2. Description of Related Art
Oil wells and wells for other fluids typically include a well casing, tubing, sucker rods and a reciprocating drive unit. A well casing is what lines the well bore and usually comprises a long string of relatively large diameter pipe interconnected by threaded couplings known as collars. Casings generally define the overall diameter and depth of a well bore. Well tubing typically comprises a long string of pipe sections whose threaded ends are also interconnected by threaded couplings. The tubing extends down through the casing and provides a conduit for conveying oil or some other fluid to the surface of the well. A submerged reciprocating pump attached to the lower end of the tubing draws the fluid from the annulus between the inside diameter of the casing and the outside diameter of the tubing, and forces the fluid up through the tubing to the surface. To operate the pump, a string of sucker rods extends through the tubing to serve as a long reciprocating connecting rod that couples the submerged pump to a reciprocating drive unit at ground level. A string of sucker rods typically includes numerous sucker rods whose ends are interconnected by a threaded rod coupling.
Wells periodically need servicing or repair. Servicing wells or drilling new ones can involve a variety of tasks that include, but are not limited to, installing or removing sections of casing, sucker rods, tubing and pumps. Such tasks are typically done using a mobile service rig, which is a truck that includes a hoist for lifting the various well components. The hoist is usually powered by a diesel engine whose speed helps determine how much power is delivered to the hoist. An operator can manually adjust the engine's speed to meet the lifting requirements of a particular job. For handling casings and other heavy loads, the engine may be run at full speed. The engine's speed may be decreased for lighter loads, such as sucker rods.
Except for some technical guidance that may be provided by the operator's supervisor, the speed of the engine or the amount of power delivered to the hoist is often left to the operator's discretion. As a result, accidents may occur when excessive power is delivered to a load. Not only can various well components be broken, but also the hoist itself can be damaged. For instance, if a transmission coupling the engine to the hoist is placed in its lowest gear while the engine is run at full speed, a tremendous amount of lifting force can be developed. Such force may exceed the rated capacity of one or more hoist components, such as the hoist's derrick, cable, or drawworks (i.e., powered drum that draws in and pays out the cable). Exceeding the rated capacity of the hoist can lead to catastrophic results.
Consequently, there is a need for a more failsafe system for ensuring that predetermined hook loads are not exceeded.